Great system, really flexible, really fast, really secure especially when hardened (which only takes a few minutes and no plugins are required unlike other CMS)...
Content creation is easy, however this often depends on how it was implemented, a bad implementation (like in any other CMS) will make content creators dislike the system, hence good implementation is key. This would also be the case in any other CMS. If implemented correctly it is way easier [and is] more enjoyable to use than wordpress. For example have had client that came from a WP as well as a typo3 background and they preferred MODX.
As a designer you are not limited to anything. No html code is generated by the CMS. Hence anything goes, we have even build apps that only output json, and the client edits the pages all the same on the backoffice. Very few limits.
Writing custom code is easy using "snippets" hence it is extremely scalable.
It is also fast, really fast, faster than most out of the box, even faster than many flat-file CMS.
Bad things are, that the learning curve is higher than in other cms, and that the community is not as big. However they are very active on slack and questions get answered very fast.
Hosting can be done by MODX directly using their cloud (for a monthly fee), or on your own server.
And it is mature, all in all it is our fav CMS out there.